Two-stage electrohydraulic valves (EHV) typically use a motor to position a flapper. The flapper, in turn, directs hydraulic fluid (which acts as an hydraulic amplifier) to either end of a valve which meters correspondingly larger flows of fluid. In this manner, a relatively small motor may position a valve which meters relatively large flows of hydraulic fluid.
Some two-stage EHV's are relatively unreliable and have high leakage rates. As a result, in some applications, a one-stage EHV, in which a metering valve is directly positioned by a motor, is required. A typical one-stage EHV has a spool valve which is directly driven by a force motor. The output of the motor is balanced by the reactional force of a spring attached to the spool valve. Since the one-stage EHV has no hydraulic amplifier, as in a two-stage EHV, it is essential that the valve encounter low friction forces to avoid hysteresis and threshold friction.
One draw back of a one-stage EHV is that a relatively large motor is required to overcome hysteresis and threshold friction. In many applications, such as aircraft fuel controls, larger motors are undesirable.
Spool valves typically have an unacceptable amount of friction inherent in their design resulting from the lack of spool straightness, the taper of the valve housing, or pressure induced side loading of the spool.
Plate valves are known to be used as part of an EHV. Typically, a force motor positions a metering plate. In a suspension type plate valve, the metering plate is suspended by a pair of arms that are disposed in a generally perpendicular manner to the motion of the valve plate. The suspension arms are also disposed, in the null position, in parallel to the fluid pressure force acting upon the metering plate. The fluid pressure causes the metering plate to open in one direction or the other, depending on the deflection of the metering plate relative to its suspension arms by the fluid pressure force acting upon the metering plate. Once the suspension arms are pushed past parallel to the direction of the fluid pressure force, the fluid pressure force pushes the metering plate even further past parallel thereby tending to open the valve even further. The unwanted opening caused by the fluid pressure tends to provide a nonlinear output relative to the input of the force motor.
Another type of plate valve disposes a metering plate between two blocks separated by a pair of spacers. The thickness of the spacers is slightly greater than the thickness of the metering plate, allowing the metering plate to slide therebetween. Metering orifices are placed in parallel, above and below the metering plate, and each block has a set of pressure and return cavities, to grossly pressure balance the plate between the two blocks to reduce contact between the metering plate and the blocks. However, manufacturing tolerances on the areas of the pressure and return cavities result in a pressure imbalance on the metering plate, and the resulting side load causes a friction force due to metal to metal contact of the metering plate and bearing and sealing surfaces. Such a valve may have friction levels as low as spool valves. However, as above, such friction forces are still too high for certain applications.
Some plate valves, in which very low friction is desired, utilize holes which pass through the metering plate. The holes are connected to the pressure supply. The fluid pressure force of the supplied fluid tends to center the metering plate between the blocks. However, the placement of the holes is critical and the amount of leakage is excessive.